1. Field of the Invention
The invention relates to a method for measuring imperfections in a digital quadrature modulator operation in which an input signal of the modulator comprises quadrature-phase I and Q channels, whereby distortions of an output signal of the modulator from an ideal one are determined and possibly corrected during a normal operation of the modulator. The method is also suited for use in a quadrature demodulator mixer, for measuring and correcting its imperfections. The invention also relates to a digital quadrature modulator and a quadrature demodulator, the imperfections of which are determined and possibly corrected with a method of the invention.
2. Background
In quadrature modulators appear a lot of imperfections, the impact of which is difficult to remove during the normal operation of the modulator. The removing of the imperfections is problematic because the characteristics of the modulator change due to carrier wave frequency, temperature, ageing, etc., and in practice such changes cannot be compensated for without regular maintenance of the modulator being required, in addition to its original tuning.
The imperfections include the following:
A. Offset voltages appearing in the I and/or Q channels in the quadrature modulator, said voltages causing carrier wave leak into the output signal.
B. Differences in amplification and thereby differences in signal levels appear in the I and Q channels, causing distortion in a constellation generated on an I/Q plane by amplitude values of the output signal, said distortion disturbing reception.
C. Deviation from a desired 90xc2x0 phase shift appears between the I and Q channels, causing crosstalk in reception between the I and Q channels and thereby disturbing reception.
D. The output power of the modulator (or the entire transmitter) can vary.
The problems are known from before and for instance the following solutions have been proposed to them:
U.S. Pat. No. 5,012,208 provides a solution to the carrier wave leak (problem A) of the quadrature modulator by a correlation, performed by analog means, of the output signal amplitude variations with signals of the I and Q channels. The apparatus in question does not, however, offer any help for solving the other problems (B, C and D).
U.S. Pat. No. 5,442,655 describes a quadrature demodulator, in which the offset voltage in the I and Q channels is measured and corrected using a two-phase procedure. In the first phase the average voltages of the I and Q channels are measured, without the time of measurement being linked to a symbol clock. The average values are then subtracted from real values and the roughly corrected values are fed into a fine correction phase. In the fine correction phase the output signals (an I/Q voltage pair) of the rough correction phase are divided into phase angle sectors on the basis of the ratio between the I and Q voltages. An average distance of signal points in a sector in relation to an axis (I or Q) is then measured from an opposite axis (Q or I) and an offset voltage is calculated as a difference of the distances in the opposite sectors. The offset voltages are then subtracted from the I and Q voltages. The publication thus describes a solution applied in a quadrature demodulator to problem A.
EP patent application 608577 A1 presents a solution, similar to the one in U.S. Pat. No. 5,012,208, to all the above mentioned problems (A, B, C, D), only its function is based on the idea that the normal operation of the modulator is interrupted for the duration of the tuning and a plurality of known test signals are run through the modulator. The apparatus in question is thus unable to correct imperfections emerging during continuous operation of the apparatus and it also requires a separate tuning to be performed in connection with the manufacturing or initialisation of the modulator.
EP patent application 0503588 A2 also describes a solution to all the above mentioned problems (A, B, C, D), only there the measurement of the modulator errors is based on two alternative implementations: (1) using specific, known test signals interrupting the normal operation, or (2) causing a slight periodic interference, separately in each adjusting parameter of the modulator, during the normal operation and examining the impact of each adjusting parameter interference on the output signal. The above solutions require either the normal operation to be interrupted or interference to be added to a normal signal, and they lead to a fairly complex implementation.
An object of the present invention is to provide a method with which in a digital quadrature modulator utilizing the method all the above mentioned problems A, B, C and D can be solved in a simple manner which can be easily, automated and made digital. This is achieved with a method of the invention characterized in that the method comprises the steps in which
from an amplitude of the output signal are taken, at a rate based on a symbol clock of the modulator, numerous momentary samples;
a direction angle of a transmission signal corresponding to the samples is divided, on the basis of data bits to be transmitted or modulator input signals, into different direction angle sectors; and
from amplitude sample deviations between the different direction angle sectors or from a nominal value are calculated the magnitudes of the distortions in the modulator operation.
The method advantageously also comprises a step wherein the measurement results of the imperfections in the modulator operation are used for adjusting the operation of the modulator by generating feedback loops, which correct all said imperfections simultaneously in an iterative manner.
The invention also relates to method for measuring imperfections in the operation of a digital quadrature demodulator in which an output signal of a quadrature mixer of the demodulator comprises quadrature-phase I and Q channels, whereby from the output signal of the demodulator quadrature mixer are determined, during a normal operation of the demodulator, offset voltages, an imbalance in the amplitudes of the I and Q channels, a quadrature error between the I and Q channels and a total amplitude error. The method comprises the steps in which
from the amplitude of the output signal of the demodulator quadrature mixer, i.e. from the amplitude of a vector formed by the I and Q channels, are taken, at a rate based on a symbol clock of the demodulator, numerous momentary samples;
a direction angle of the demodulator output signal vector corresponding to the samples is divided, on the basis of the voltages of the I and Q channels, into different direction angle sectors; and
from amplitude sample deviations between the different direction angle sectors or from a nominal value are calculated the magnitudes of the distortions in the demodulator operation.
The method advantageously also comprises a step wherein the measurement results of the imperfections in the demodulator operation are used for adjusting the operation of the demodulator by generating feedback loops, which correct all said imperfections simultaneously in an iterative manner.
The invention is based on the monitoring of the output amplitude of the modulator/demodulator and, above all, on the observation that all the above mentioned imperfections (A, B, C and D) can be measured, using simple inference rules, on an I/Q plane (from what is known as a constellation figure) from a large quantity of momentary samples taken from a modulated signal at a rate based on a symbol clock.
As a solution according to the invention to problem A, DC offsets of the I and Q channels are determined from a difference in the amplitudes of the samples inside two opposite direction angle sectors in relation to a respective axis.
As a solution to problem B, the differences in the signal levels of the I and Q channels are determined on the basis of how much the sum of the amplitudes of the samples of one channel inside two opposite direction angle sectors around a respective axis increases or the decreases in relation to the other channel.
As a solution to problem C, a phase error in a phase difference of 90 degrees in the carrier waves of the I and Q channels is determined on the basis of how much the sum of the amplitudes of the samples inside direction angle sector pairs of opposite directions around one of two axes which are in an angle of 45 degrees to the I and Q axes increases or the reduces in relation to the other axis.
As a solution to problem D, a change in the output power of the modulator/demodulator is determined based on the amplitude of the samples.
In order to simplify the implementation into practice of the method, it is advantageous that the sample group is reduced in such a way that only samples relating to symbols that are known to have the same nominal amplitude are accepted in the calculation.
The invention also relates to a digital quadrature modulator comprising means for modulating quadrature-phase I and Q signals to generate an output signal that comprises quadrature-phase I and Q components. According to the invention, the quadrature modulator is characterized in that the modulator further comprises
means for taking momentary amplitude samples from the output signal of the modulator, at.a rate based on the symbol clock of the modulator;
means for classifying a symbol to be modulated at a particular time into a particular direction angle sector;
means for linking the amplitude samples of the output signal to the direction angle sector corresponding to the symbol to be transmitted at a particular time; and
means for comparing the amplitudes of the samples belonging to each direction angle sector with other direction angle sectors or with an ideal value, for determining from the output signal of the modulator a local oscillator carder wave leak, an amplitude imbalance between I and Q channels, a quadrature error between the I and Q channels and an amplitude error.
The modulator advantageously further comprises means for correcting the adjusting parameters of the modulator, in response to the predetermined distortions, by forming feedback loops, which correct all said imperfections simultaneously in an iterative manner.
The invention also relates to a digital quadrature demodulator comprising a quadrature mixer for generating analog quadrature-phase I and Q output signals from a quadrature modulated input signal. The demodulator is characterized in that it comprises
means for taking momentary samples from the I and Q output signals of the quadrature mixer, at a rate based on a symbol clock of the demodulator;
means for calculating the total amplitude samples of the output signal from the I and Q samples;
means for classifying the direction angle of the symbol received at a particular time to a particular direction angle sector;
means for linking the amplitude samples of the output signal of the quadrature mixer to the corresponding direction angle sector; and
means for comparing the amplitudes of the samples belonging to each direction angle sector with the ideal value or other sectors, for simultaneously determining the offset voltages, the amplitude imbalance between the I and Q channels, the quadrature error between the I and Q channels and the total amplitude error in an iterative manner.
The demodulator advantageously further comprises means for correcting the adjusting parameters of the modulator, in a manner depending on the predetermined distortions, by forming feedback loops which correct all said imperfections simultaneously in an iterative manner.
In order to simplify the structure of the modulator or demodulator of the invention, it is advantageous that it further comprises means for selecting the amplitude samples that have the same nominal amplitude.
In an apparatus of the invention all the above mentioned imperfections are continuously detected during the normal operation of the modulator, without any measures by the user being required. In its digital form of implementation, the apparatus of the invention is almost entirely also easy to integrate into the same circuit with other baseband parts of the modulator. After the imperfections have been detected, they can also be automatically and continuously corrected during the normal operation of the modulator.
According to the above, a fundamental discovery of the invention is that the above distortions in constellation can be measured by studying only the momentary amplitude of the output signal of the modulator (transmitter)/demodulator mixer in a manner which depends on the symbol to be transmitted/received. The practical implementation of the method of the invention becomes thus easier and more accurate since it is restricted to cases in which an ideal constellation of the modulation comprises several ( greater than 2) points with one and the same amplitude. Such modulations include different versions of PSK (at least 3, i.e. in practice 4 phase modes) and QAM and all constant amplitude modulations (e.g. CPM) generated using a quadrature modulator. In connection with most of the modulations, sampling can be performed at the rate of the symbol clock, although some modulations can require the amplitude to be measured also between the symbol clock periods. In a case of constant amplitude, the non-linearity of the power measurement cannot cause errors in the correction of the imperfections to be performed. If the invention is implemented without such constant amplitude points, an equal accuracy in the correction of thee imperfections is not achieved.